I'm not saying that it is long-term effects - lactic acid production is a short term thing that, until it is metabolized causes pain in the muscles due to respiration. My statement is that a body has a certain amount of energy available to it, and if she can use more than her body has then she breaks conservation of energy or there is some external magical force. What I am saying is that she uses her powers to force her mitochondria to anaerobically respire, causing a big surge in work done, but burning ATP fast and inefficiently.